1. Field of the Invention
This invention relates to a process for catalytic cracking hydrocarbons comprising contacting a hydrocarbon feedstock at an elevated temperature in a cracking zone with a catalyst comprising a mixture of a conventional crystalline aluminosilicate zeolite, i.e., a FC (fluid cracking) catalyst and a hybrid [Al,B]-zeolite. The cracked product derived from the process of this invention contains increased amounts of C.sub.3 -C.sub.5 olefins, particularly C.sub.5 olefins. This invention is particularly applicable to fluid catalytic cracking (FCC) processes.
The recent passage of the Clean Air Act requires the petroleum industry to develop "clean" gasoline to meet future reformulated gasoline requirements. Fluid Catalytic Cracking Units, i.e., FCC units, will play an important role in achieving that goal by producing light olefins for alkylation and etherification processes. The increased olefin production provided by the process of this invention will further enhance FCC as a process to produce an environmentally acceptable fuel.
2. Prior Art
A number of fluid catalytic cracking processes are known in the art. State of the art commercial catalytic cracking catalysts for these processes are highly active and possess high selectivity for conversion of selected hydrocarbon charge stocks to desired products. With such active catalysts it is generally preferable to conduct catalytic cracking reactions in a dilute phase transport type reaction system with a relatively short period of contact between the catalyst and the hydrocarbon feedstock, e.g., 0.2 to 10 seconds.
The control of short contact times, optimum for state of the art catalysts in dense phase fluidized bed reactors is not feasible. Consequently, catalytic cracking systems have been developed in which the primary cracking reaction is carried out in a transfer line or riser reactor. In such systems, the catalyst is dispersed in the hydrocarbon feedstock and passed through an elongated reaction zone at relatively high velocity. In transfer line reactor systems, vaporized hydrocarbon cracking feedstock acts as a carrier for the catalyst. In a typical upflow riser reactor, the hydrocarbon vapors move with sufficient velocity to maintain the catalyst particles in suspension with a minimum of back mixing of the catalyst particles with the gaseous carrier. Thus development of improved fluid catalytic cracking catalysts has led to the development and utilization of reactors in which the reaction is carried out with the solid catalyst particles in a relatively dilute phase with the catalyst dispersed or suspended in hydrocarbon vapors undergoing reaction, i.e., cracking.
With such riser or transfer line reactors, the catalyst and hydrocarbon mixture passes from the transfer line reactor into a first separation zone in which hydrocarbon vapors are separated from the catalyst. The catalyst particles are then passed into a second separation zone, usually a dense phase fluidized bed stripping zone wherein further separation of hydrocarbons from the catalyst takes place by stripping the catalyst with steam. After separation of hydrocarbons from the catalyst, the catalyst is introduced into a regeneration zone where carbonaceous residues are removed by burning with air or other oxygen-containing gas. After regeneration, hot catalyst from the regeneration zone is reintroduced into the transfer line reactor into contact with fres hydrocarbon feed.
Commercial cracking catalysts for use in a fluidized catalytic cracking process which have been developed to be highly active for conversion of relatively heavy hydrocarbons into naphtha, lighter hydrocarbons and coke demonstrate selectivity for conversion of hydrocarbon feed, such as vacuum gas oil, to a liquid fuel fraction at the expense of gas and coke. One class of such improved catalytic cracking catalysts includes those comprising zeolitic silica-alumina molecular sieves in admixture with amorphous inorganic oxides such as silica-alumina, silica-magnesia and silica-zirconia.
U.S. Pat. No. 5,106,485 to Himpsi, et al., teaches a process for the catalytic cracking of gas oils to obtain gasoline wherein the gas oil is contacted in a fluid catalytic cracking unit at elevated temperatures with a mixture of zeolite Y and zeolite L in an inorganic matrix and wherein the zeolite L is present in an amount ranging from 1 to 10 wt. % based on the total catalyst composition.
U.S. Pat. No. 5,037,531 to Bundens, et al., teaches a process for catalytically cracking a hydrocarbon feedstock in which a zeolite-modified cracking catalyst having deposited thereon a treating agent selected from gallium and a gallium compound is employed.
U.S. Pat. No. 5,059,302 to Weinberg, et al., discloses a fluid catalytic cracking process in which the hydrocarbon feedstock is contacted with a particulate sorbent, such as calcined clay, and a particulate FCC catalyst, such as a Y zeolite zequentially in the same FCC riser followed by separation of the commingled spent catalyst and sorbent particles from the vapor and the subsequent primary partial regeneration of the spent sorbent particles and catalyst particles in an oxygen deficient burning zone.
U.S. Pat. No. 4,656,016 to Taramasso, et al., describes a process for preparing a synthetic, silica-based boron modified material selected from the group consisting of crystalline silica modified through the introduction of boron into the crystalline latice as a replacement element for silicon.
U.S. Pat. No. 4,269,813 discloses a crystalline borosilicate having the composition in terms of mole ratios of oxides: EQU 0.9.+-.0.2 M.sub.2n O:B.sub.2 O.sub.3 :YSiO.sub.2 :ZH.sub.2 O
where M is at least one cation, n is the valence of the cation, Y is a value within the range of 4 to about 600, and Z is a value within the range of 0 to about 160 and providing a specific X-ray diffraction pattern. The borosilicate is used to catalyze various processes, such as isomerization, disproportionation, and transalkylation.